1. Field of the Invention
The present invention relates to a thermal recording apparatus utilizing a thermal energy of heating elements provided on substrates for forming a desired visual image on a recording medium oppositely disposed. More particularly, the present invention relates, to a thermal recording apparatus which uses a thermal head composed of a plurality of substrates having a plurality of heating elements arranged in line on each of the substrates for the purpose of providing a wider recording width, to thereby improve a recording quality in recording the visual image on the recording medium.
2. Description of the Prior Art
A thermal printer, thermal transfer printer, facsimile apparatus, and XY plotter have been in practical application as thermal recording devices using a thermal head having a plurality of heating elements arranged in line along the direction of recording width. In these thermal recording apparatus, many heating elements record a specific recording width and are generally deposited in a line on one substrate by a metallizing process, and a thermal head with the least possible deviation of a resistance value of each heating element is adopted to prevent occurrence of a recording density variation. Some of these recording devices require a thermal head having a wide recording width to accomplish their functions. It is, however, very difficult to build, on one substrate, a thermal head corresponding to such a recording width as A2, A1, and A0 sheet sizes specified by ISO because of dimensional limit and yield in the thermal head manufacturing equipment. For example, the dimensional limit of a metallizing oven interior in the manufacturing process is one limitation on size of a thermal head.
In the meantime, thermal transfer color recording adopted in a prepress application is for recording a quality image of ISO or ANSI size within a region of specific width at the central part of the recording chart, and for recording a positioning mark called a crop mark as an auxiliary information in the recording width B' part on either side of the recorded image. Generally, however, the recording of auxiliary information requires little uniformity of recording density as in the image recording area. It is, however, necessary to use a thermal head further extended by at least 2B' in addition to the recording width C' of an actual image in order to record these auxiliary information.
It is from this reason that in a prior art thermal recording devices requiring a wide recording width, a thermal head capable of executing recording in a wide recording width is realized by jointing in one row a plurality of substrates having heating elements of a relatively narrow recording width. To obtain a thermal head having a row of heating elements of a length W, a substrate having a heating element row of the length D and a substrate having a heating element row of the length W-D are connected. At this time, the lengths W and D of the heating element rows have no relation to the width of an image to be recorded; the heating element rows are connected in the relation of D=W/2. As a result, at a connection between the two heating element rows on adjacent substrates, a discontinued deviation of the resistance value is likely to occur between the heating elements, resulting in an increased deviation between adjacent resistance values and accordingly a noticeable variation in the recording density.
This variation in the recording density causes a streak to occur nearly at the center of the image along the direction of travel of the recording chart, resulting in a deteriorated image quality in the case of the thermal recording device in which the image is outputted with a consecutive gradation particularly as in a sublimation type thermal transfer color printer.